1. Filed of the Invention
The present invention relates to an overwrite capable magnetooptical recording method.
2. Related Background Art
Conventionally, a magnetooptical memory is known as an erasable optical memory. The magnetooptical memory has merits such as high-density recording, non-contact recording/reproduction, and the like as compared to a magnetic recording medium using a magnetic head. However, in order to record information on such a magnetooptical memory, in general, an already recorded portion must be erased (magnetized in one direction) before recording.
Thus, a method of independently arranging a recording/reproduction head and an erasing head, a method of performing recording while modulating a magnetic field which is applied simultaneously with radiation of a continuous laser beam, and the like have been proposed. In the former recording method, the apparatus becomes large in scale, resulting in high cost. In the latter recording method, high-speed modulation is disturbed when a large magnetic field is applied. When a magnetic head is arranged near a medium surface to achieve high-speed modulation, the rotating medium and the magnetic head contact each other to cause a head crash, or the magnetic head is easily influenced by dust attached to the medium surface. As a result, the original merits of magnetooptical recording may be lost.
In order to solve these problems, a method for realizing overwriting by the following two types of recording methods using a two-layered perpendicular magnetic film which comprises a magnetic layer having a large coercive force and a low Curie temperature, and a magnetic layer having a small coercive force and a high Curie temperature has been proposed. In one recording method, after the direction of magnetization of only the layer having the small coercive force is aligned in one direction by applying a relatively large magnetic field, recording is performed by inverting the direction of magnetization of the aligned layer by applying large laser power. In the other recording method, recording is performed by aligning the direction of magnetization of the layer having the large coercive force and the low Curie temperature to the direction of magnetization of the layer having the small coercive force by applying small laser power without inverting the direction of magnetization of the layer having the small coercive force and the high Curie temperature. However, in this method, a recording state wherein a magnetic wall is present between the two magnetic layers may pose a problem associated with the stability of a recorded bit. Also, a magnetic field generating unit for aligning the direction of magnetization of the layer having the small coercive force becomes large in scale.
In addition, a method of reversing the direction of magnetization of a portion irradiated with a laser beam by a surrounding reversing magnetic field, as disclosed in, e.g., Japanese Laid-Open Patent Application No. 3-127347, has also been proposed. However, a method of performing recording by controlling the balance between the magnitudes of a recording bias magnetic field and a reversing magnetic field generates many noise components, and it is difficult to put this method into practical applications.